A valve is a device that regulates the flow of a substance. Valves are produced in a variety of different styles, shapes and sizes. Typically, valves are used for gases and liquids. However, valves are also used on solids capable of flow, slurries or any other substance capable of flow. Valves are used in almost every industry having a substance that flows.
Gate valves are generally comprised of a valve body having a central axis aligned with inlet and outlet passages, and a space between the inlet and outlet passages in which a slide, or gate, may be moved perpendicular to the central axis to open and close the valve. In the closed position, the gate surfaces typically seal against sealing rings which surround the fluid passage through the valve body. Gate valves have been used for centuries to control the flow of a great variety of fluids. Often the fluid to be controlled by the gate valve is under pressure. In the petroleum industry, gate valves are used along piping at various locations, and in particular are used in piping referred to in the petroleum industry as a Christmas tree, which is used as part of a drilling operation.
Gate valves may be operated manually or automatically. One method to automatically operate a gate valve is to use an actuator. An actuator is a mechanical device for moving or controlling a mechanism or system. When an actuator is used in a gate valve, the actuator is typically linked to a stem to repeatedly move the valve gate between open and closed positions.
Actuators to open and close the gate valves may include manual operators, diaphragm-type operators, and hydraulic operators. The actuator may include a bonnet assembly, which interconnects the valve body and the valve gate, and a bonnet stem which is movable with the gate via an operator. The operator typically has a maximum force capability for applying to the bonnet stem. It is sometimes desirable to provide additional opening/closing power on a temporary basis without having to remove the original operator. It is also desirable that the same operator be adaptable to various control accessories, such as a mechanical override, hydraulic override, heat sensitive lock open device, block open cap, electrical limit switch and/or other electrical accessories.
In the use of pneumatic actuators, typically a pressure source is needed to provide pressure against the piston such that the piston is pushed in a proximal direction forcing the operator shaft and the gate of the gate valve to move in a proximal direction. Typically this results in closure of the gate valve in fail open types of valves. The amount of pressure needed to move a gate valve in a proximal direction is related to both the pressure entering the pressure chamber of a pneumatic piston actuator and the surface area of the piston.
However, in many applications, a strong pneumatic pressure source or a dedicated pneumatic pressure source to keep a gate valve closed (if designed to fail open for example) may not be available. It is also problematic when a pressure inlet hose needs to be diverted to another piece of equipment due to lack of pressure lines or failure of such other equipment.
What is needed is a method to allow a gate valve to be kept in an open position or closed position under pressure without requiring the full amount of pressure necessary to move the gate valve into the open position or the closed position.